iio: make iio_device_get_drvdata take a const struct iio_dev *.

As this just calls dev_get_drvdata underneath which is happy with
a const struct device * we should change and avoid potentially
casting away a const in order to then put it back again.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>